Support an external postgresql config in the Docker image

contrib/docker/conf/homeserver.yaml

@@ -46,8 +46,17 @@ listeners:
 
 ## Database ##
 
-{% if SYNAPSE_DB_HOST %}
-
+{% if SYNAPSE_DB_PASSWORD %}
+database:
+  name: "psycopg2"
+  args:
+    user: "{{ SYNAPSE_DB_USER or "matrix" }}"
+    password: "{{ SYNAPSE_DB_PASSWORD }}"
+    database: "{{ SYNAPSE_DB_DATABASE or "matrix" }}"
+    host: "{{ SYNAPSE_DB_HOST or "db" }}"
+    port: "{{ SYNAPSE_DB_PORT or "5432" }}"
+    cp_min: 5
+    cp_max: 10
 {% else %}
 database:
   name: "sqlite3"